Typical modern dimmable fluorescent lamp systems include a high-frequency resonant ballast inverter operating from a dc input voltage source and supplying approximately sinusoidal high-frequency ac current to the fluorescent lamp(s). The dc input voltage to the ballast is typically obtained by rectifying an ac utility voltage or, alternatively, directly from a dc source such as a battery. By operating the resonant ballast inverter above its natural resonance, the output characteristics of the inverter resemble an ideal ac current source, and the dimming function is achieved by increasing the inverter switching frequency which, in turn, reduces the lamp current. In addition to regulating light output of the lamp, the ballast inverter also provides high output voltage for starting the lamp(s) having an amplitude which is usually two to three times higher than the lamp voltage during normal operation.
To maximize efficiency, it is desirable to have zero-voltage switching (ZVS) operation of the inverter switching devices which greatly reduces switching losses. (ZVS is switching a device with zero voltage thereacross.) On the other hand, if the same resonant inverter is operated below resonance, ZVS operation is lost and the devices of the inverter are hard-switched, resulting in relatively high switching losses. Therefore, in high-frequency ballasts with a wide dimming range, i.e., 100% to about 1% full light output, it is desirable to enable ZVS operation throughout the entire dimming range. It is also desirable to maintain ZVS operation and the full dimming range with a wide input dc voltage range, e.g., two-to-one. Still further, it is desirable to achieve the aforementioned features over a relatively narrow switching frequency range in order to lower switching losses, reduce likelihood of interference with other equipment, and to reduce resonant switching stresses.
Accordingly, it is desirable to provide a control for a dimmable fluorescent lamp ballast which maintains operation above resonance over a wide dc input voltage range and throughout the entire dimming range, while maintaining a relatively narrow switching frequency range. In addition, because ballast inverter operation below resonance results in relatively high switching losses and could cause overheating and even failure of switching devices, it is desirable to have a protective feature for detecting and preventing prolonged operation of the ballast below resonance. The large amplitude of the output voltage waveform during starting is also potentially damaging to resonant components at the output of the ballast inverter such that some type of overvoltage protection is desirable. Such overvoltage protection should not interfere with normal starting and running operation of the ballast, but should prevent the output voltage from exceeding a predetermined value by shutting down operation of the inverter. (It should be noted that during ZVS operation, the resonant ballast inverter is immune to a short circuit condition at the output, making it unnecessary to protect a system operating with ZVS against this failure mode.)